From DE 199 12 443, an electrical assembly with at least one semiconductor element is known, by which, for example, a conductor track formed as a conducting-lead frame is welded to a metallic heat sink of a power semiconductor unit by means of a fillet weld. Through this, a secure electrical contact is reached and, at the same time, the lead-away of the dissipated heat of the power semiconductor unit to the conductor track is assured. Moreover, since the heat sink of a power semiconductor unit is ordinarily joined to a connector contact of the assembly, there is the advantage that the connector contact in question does not additionally still have to be connected electrically with the conductor track in question.
When welding a power semiconductor unit to a conductor track, it is necessary to bring as little heat as possible to the heat sink of the power semiconductor unit during the welding procedure, as otherwise there is the danger of damage or destruction of the semiconductor chip placed on the heat sink.
The welding procedure carried out up to now by the applicant is more closely explained briefly in the following, by means of the representations in the FIGS. 1a and 1b to illustrate the difficulties connected with it.
As represented in FIG. 1a, a first welding part 1, which can be for example a conductor track in the form of a conducting-lead frame, is brought into contact with its lower surface at the top of a second welding part 3. In the case of the second welding part, it can be for example the metallic heat sink of a power semiconductor unit.
The welding should take place, for example, by means of laser welding on the right edge of the first welding part 1. In this regard, the welding energy is introduced to the first welding part at its edge area by means of a laser beam 5. Since it concerns the heat sink of a power semiconductor unit for the second welding part 3, one must make sure that the welding energy is introduced only in as small a dosage as possible to the second welding part 3, in order to prevent damage or destruction of the semiconductor chip. As a result, the laser beam is so positioned with respect to the first welding part 1 that the laser beam, which has a diameter corresponding to the bead width wo lies with its entire bead of diameter wo in the range of the top edge of the first welding part 1. After experiments, it has been established that the angle which the laser beam has with the contact area between the first and second welding part 1, 3 should preferably lie in the area of 50 to 80 degrees. If the angle which the laser beam 5 makes with the contact surface were to be 90 degrees or more, then the desired fillet weld would not develop between the edge of the first welding part 1 and the top of the second welding part 3.
The completed weld joint is shown in FIG. 1b. The top corner area or, respectively, the edge area of the first welding part is melted by means of the supply of the welding energy of the laser beam 5. Since it was not permitted in this case to direct the laser beam directly into the corner area between the edge of the first welding part 1 and the top area of the second welding part 3, the entire top edge or, respectively, edge area of the first welding part 1 must, consequently, be melted. In the process, the welding energy must be so great and sustained for so long, until not only the entire edge area of the first welding part 1 is melted, but also, a sufficient area of the second welding part 3, which is adjacent to the melted edge of the first welding part, is melted as well and fused with the material of the first welding part 1.
The completed weld pattern of the fillet weld is shown in FIG. 1b. As a result of the required melting of the top edges and the welding energy used, which has to be as low as possible, there is frequently a welding pattern which is optically somewhat unappealing since a relatively large material flow is necessary.
Furthermore, the positioning of the welding parts relative to the laser beam, as a consequence of the perpendicular edge of the first welding part, is very critical. Even minimal displacement of the first welding part 1 in FIG. 1a to the left would cause the laser beam to register its welding energy closer and closer to the sensitive second welding part 3 or even upon it. That is absolutely to be prevented.
Furthermore, small position changes of the first welding part 1 relative to the laser beam 5 lead to strong different material flows during melting of the edge area of the first welding part 1. The production of an optically appealing, uniformly shaped welding bead is therefore difficult.